Spectrophotometer for measuring thickness or weight of water-containing coatings

ABSTRACT

The thickness and/or weight of a water-containing composition is measured by an infrared spectrophotometer wherein the attenuation of radient energy at a measuring wavelength which is substantially absorbed by the coating composition is compared with the attenuation of radient energy at a second reference wavelength not substantially absorbed by the coating composition.

D United States Patent [151 3,661,462 Natens 51 May 9, 1972 54]SPECTROPHOTOMETER FOR [56] References Cited MEASURING THICKNESS ORWEIGHT OF WATER-CONTAINING COATINGS UN'TED STATES PATENTS 3,413,482 I]1968 L' ..250 43.5 [721 lnvemo Yves Nam, Bmhem Belg'um 3 441 351 4/1969B e l l et al ..356 ll 75 [73] Assignee: Gevaert-AGFA N.V., Mortsel,Belgium Primary E.\'aminerRonald L. Wibert [22] Ffled: 1970 AssistantExaminer-V. P. McGraw [21] Appl.No.: 31,189 Altorney-William J. Daniel[57] ABSTRACT [30] Foreign Application Priority Data The thicknessand/or weight of a water-containing composi- Apr. 23, 1969 Great Britain..20,766/69 {ion is measured by an i f d spectrophotometer wherein theattenuation of radient energy at a measuring wavelength [52] U.S. Cl.356/51, 250/435 R, 250/226, which is substantiany absorbed by thecoating composition is 356/96, 356/188, 35'6/204 compared with theattenuation of radient energy at a second [51 Int. Cl ..G0lm 21/34, G013/42 reference wavelength not Substantially absorbed by the coat. [58]Field of Search ..356/51, 96-98, ing composition.

9 Claims, 8 Drawing Figures PATENTEDMM 9 m2 SHEET 1 or 4 FIG. 1

FIG. 2

SPECTROPI-IOTOMETER FOR MEASURING THICKNESS OR WEIGHT OFWATER-CONTAINING COATINGS The present invention relates to aspectrophotometer wherein the measurement of the spectral absorption ofa medium occurs by the comparison of the attenuation of radiant energyat a wavelength which is subject to absorption by said medium, with theattenuation of radiant energy at a wavelength which is not, or almostnot, subject to absorption by said medium. The invention relates inparticular to an infrared waterband meter for measuring the coat weight,or the thickness, of layers which are applied to flexible webs, e.g. inthe manufacture of wrapping or, in particular, of photographic and otherrecording materials.

It is known that water shows several absorption maxima in the infra-redspectrum. For measurements of water layers with a thickness in the rangeof to 100 pm, the absorption at a wavelength of 1.96 pm offers aninteresting compromise between the sensitivity of the measurement andthe difficulties in the construction of the apparatus. Since accuratemeasurements cannot be performed at this wavelength on account ofunreliabilities in the radiation sources and the detectors, the ratio ofthe attenuation of radiation at 1.96 pm is determined in practice versusthat of one or more wavelengths in the range of said water absorptionwhich, however, are not or not substantially influenced by the presenceof water.

When the measurement occurs according to the compensation method,wherein the position of a measuring wedge in the light beam, which isreflected or transmitted by the sample to the detector, is controlled bya servomotor so that the radiant intensity impinging on the detector isconstant at equilibrium of the system, deficiencies in the linearity ofthe response of the detector are overcome.

Apparatus operating according to the mentioned compensation methodsuffer, however, from two serious drawbacks.

First, their response is relatively slow because of the displacements ofthe measuring wedge involved before the equilibrium is reached. Second,the reliability is not very good since these apparatus comprise adelicate optical-mechanical beam-split system, wherein two optical beamsare treated separately and in consequence thereof can be disturbed in adifferent way.

According to the present invention an infrared waterband meter, or moregenerally, a spectrophotometer of the type described, is provided, whichhas a simple optical system and which has a quick response, say lessthan 0.02 sec.

The spectrophotometer according to the invention wherein the measurementof the spectral absorption of a medium occurs by the comparison of theattenuation of radiant energy at a first wavelength hereinafter calledthe measuring beam which is subject to substantial absorption by themedium with the attenuation of radiant energy, at a second wavelengthcalled the reference beam which is substantially not subject toabsorption thereby, comprises:

a. a radiant energy detector,

b. a radiant energy source for directing radiant energy in at least thementioned first and second wavelengths towards said medium so that saidenergy after impingement thereon is received on said detector,

. filter means in the radiation path from the source to the detector andarranged to alternatively transmit radiation at said first wavelengthand radiation at least at said second wavelength,

d. a measuring wedge mounted for introduction in the radia tion beam asradiation in one of said wavelengths is transmitted to the detector andwhich progressively alters the amount of transmitted radiant energy asit moves through the radiation beam,

e. means for producing a first electric signal which is proportional toradiant energy received on the detector at the wavelength which is notattenuated by said photographic wedge and means for storing said signal,

f. means for producing a second electric signal which is proportional toradiant energy received on the detector at the wavelength which isprogressively altered by the said measuring wedge,

g. means for comparing said second electric signal with said first oneand for detecting the equality of both said electric signals, and

h. means for producing at said equality an indication of the positiontaken by the measuring wedge at said moment.

The term wavelength as used in the present description may denote thepeak of a narrow wavelength range, e.g. a measuring beam at 1.96 pm or areference beam at 1.71 pm, but said term may also apply to a widerwavelength range, e.g. a reference beam at a wavelength band from 1.79to 2.17 am.

The term radiation source" covers the element as such which producesradiant energy, but it includes also additional filter means which maybe provided in order to limit the wavelength or wavelengths whereinradiant energy is provided.

By the statement that a filter transmits radiation at a certainwavelength it is meant that said filter does not substantially transmitradiation at other wavelengths or at least does not transmit radiationat wavelengths which would interfere with other wavelengths used in theperformance of the measurement according to the invention.

In the comparison of the said two electric signals, the radiation whichis transmitted when the measuring wedge is in the radiation path, may bethe measuring as well as the reference radiation beam.

Thus, in one system the electric signal which is proportional to thereference radiation beam is stored and compared with the electrical rampfunction which is generated by the measuring wedge passing through themeasuring radiation beam.

In the other system, the electric signal which is proportional to themeasuring radiation beam is stored and compared with the ramp functionwhich is produced when the measuring wedge passes through the referenceradiation beam.

The measurement which is carried out by means of the apparatus accordingto the present invention can be done in transmission or by reflection,and in this respect the statement that the radiant energy is received onthe detector after impingement on the medium applies to the case whereinsaid energy is reflected by the medium as well as to the case whereinthe energy is transmitted thereby.

Although the invention has been particularly developed in connectionwith the thickness measurements of silver halide and antistress layersin the manufacturing of light-sensitive photographic material, it willbe apparent that the invention is not limited to this particularapplication and that many other applications of the apparatus accordingto the invention are possible.

The invention is described hereinafter with reference to the drawingswherein:

FIG. I is the optical arrangement of the apparatus according to theinvention for carrying out the measurement by reflection.

FIG. 2 is the filter disk for carrying out the measurement at twoselective wavelengths.

FIG. 3 is a diagram which shows the output current I of the photo-cellas a function of the angular position of the filter disk.

FIG. 4 is a diagram showing the impulses E produced as a function of theangular position of the filter disk.

FIG. 5 is a diagram which shows the output current I of the photo-cellfor a measurement occurring at one selective wavelength and at a broaderband.

FIG. 6 is the electric circuit of the apparatus.

FIG. 7 is the optical arrangement for carrying out the measurement intransmission.

FIG. 8 is the filter disk for carrying out the measurement at oneselective wavelength and at one broader band.

Labels for numerical designations in FIG. 6.

19 magnetic head 22 photocell 30 pulse train from head 19 32 pulseformer 33 gate 34 monostable multivibrator 35 rectangular output pulse36 counter 37 diode matrix 38 diode matrix 39 diode matrix 41 analogswitch 42 preamplifier 43 sample and hold circuit 44 normally closedelectronic switch 46 voltage comparator 47 gate 48 indicator 51photo-cell housing 52 heater 53 sensing element 54 controller.

In the arrangement shown in FIG. 1 the radiant energy from a source 10,in the present case a conventional incandescent low voltage bulb of Wattfed by a stabilized power supply, is focused by a converging lens 11onto a mirror 12. An infrared filter 13 transmits a wavelength rangefrom 1.79 to 2.17 pm. The beam which is reflected by the mirror 12 isfocused on a mirror 14 by the lens 15. The spectral composition of theradiation beam between both said mirrors is modified by a filter disk 16which is mounted at one extremity of the shaft of an electric motor 17.At the other extremity the motor shaft is provided with a narrow drum 18the periphery of which is provided with a length of magnetic tape onwhich a magnetic signal has been recorded so that upon rotation in frontof a magnetic head 19 electric impulses are generated. Further detailson the filter disk are given hereinafter with reference to FIGS. 3 and4.

The radiation beam which is directed by the mirror 14 onto thewater-containing layer 20, e.g. a silver halide gelatin layer which hasjust been coated onto a paper or film support, is partly reflected andfocused by a concave mirror 21 onto the lead sulfide photo-cell 22. Aninfra-red filter 23 limits the response of the photo-cell to radiationwith a lower wavelength of 0.9 pm. The upper wavelength limit is focusedby the characteristics of the photo-cell itself at longer wavelengths.The photo-cell 22 is actually mounted in a housing 51, the temperatureof which is kept thermostatically at 55 C. by means of a heater 52, anda sensing element 53 driving the controller 54 (FIG. 6).

The filter disk 16 is shown in detail in FIG. 2. It is composed of acircular glass disk onto which the following filters are provided.

An interference filter 25 which extends over about angular degrees ofthe disk, which transmits a wavelength of 1.71 um, and the boundaries ofwhich are determined by the radiation absorbing small sectors 26 and 27.

An interference filter 28 which occupies the remaining angular part ofthe disk and which transmits radiation at a wavelength of 1.96 pm. Oversaid filter 28 a continuous grey wedge 29 is provided the density ofwhich increases linearly as a function of the angular position. Therepresentation of said wedge in the drawing is merely diagrammatical andactually the width of the wedge is constant according to the radius ofthe disk.

The output of the arrangement according to FIG. 1 is shown in thediagrams of FIGS. 3 and 4.

In the diagram of FIG. 3 the output current I of the photocell isplotted versus the angular position a of the filter disk. For thepurposes of the present description the radius which is indicated by thenumeral 31 in FIG; 2 constitutes the zero angular position of the disk,the rotation of the disk occurring in clockwise direction.

The intensity of radiation at 1.71 pm which is received on thephoto-cell is represented by the first part of the curve while theintensity of radiation at 1.96 pm is represented by the sloping part ofthe curve.

The maximum current at 1.7 1 m, i.e. the reference signal, is determinedby the characteristics of the optical system, the

intensity of the light source, the sensitivity of the photo-cell and theoccasional absorption by the medium to be measured, in the present casethe layer 20.

The slope of the current curve at 1.96 am, called hereinafter themeasuring signal, is determined by the parameters mentioned hereinbeforein connection with the reference signal and by the I.R.-absorption ofthe layer 20.

The pulse train 30 which is produced by the magnetized drum 18 is shownin the diagram of FIG. 4. The starting position of a pulse series ismarked by a small zone wherein no impulses are provided, in the presentexample the angular position composed between 0 and 15 angular degrees.

The electric circuit for carrying out the measurement is.

represented in FIG. 6.

The electric signals which are produced in the magnetic playback head 19are amplified and given an appropriate shape in the pulse former 32. Thepulse train thus formed is passed to a gate circuit 33 which iscontrolled by a monostable multivibrator 34. The multivibrator in itsturn is controlled by the impulses from the pulse former 32 and acts asa pulse stretcher so that no output is provided during the pulse train,but that a marked rectangular output pulse 35 is produced during thetime the impulse train is held off. This time corresponds almost to theangular position of the disk comprised between 0 and 15 angular degrees.The leading edge of the signal 35 controls the closing of the gate 33,whereas the trailing edge thereof controls the opening of the gate andalso the resetting of the BCD counter 36 over line 56.

Three diode matrices 37, 38 and 39 are connected to the outputs of thecounter 36. At the 10th impulse received from the counter 36 the matrix37 controls the closing of the flipflop circuit 40 whereas the matrix 38controls the opening of the circuit 40 at the 15th impulse.

The output signal of the flip-flop circuit controls the analogue switch41 in such a way that during the interval comprised between the 10th andthe 15th impulse the switch takes the position as shown in broken lines.The output signal of the photo-cell 22, which has been amplified by apreamplifier 42, is fed to the sample-and-hold circuit 43 over theclosed electronic switch 44.

After the 15th impulse, the switch 41 is put in the position as shown ina drawn line so that over an amplifier 45 with adjustable feedback loopthe signal of the photo-cell is connected to the voltage comparatorcircuit 46. At the same time the contact of switch 44 was broken so thatthe reference signal at 1.71 pm remains stored in the circuit 43.

The third matrix 39 controls the opening of the gate circuit 47 at thereceiving of the 30th impulse, so that up from said moment the impulsesfrom the gate circuit 33 are counted and displayed in the circuit 48. Atthat very moment the filter transmitting the 1.9 pm wavelength hasentered the radiation path, see the angular position of about 45 in FIG.3, so that the proper measuring starts. In the voltage comparator 46 theintensity of the measuring signal at 1.96 pm is continuously comparedwith the intensity of the reference signal at 1.71 m and as equality isreached, in the present case at an angular position of the filter diskat 250, an impulse is produced by the trigger which controls the closingof the gate 47. The number of impulses which is displayed by the device48 corresponds to the rotation of the filter disk after the measuringwedge 29 has entered the radiation beam, and gives an unequivocalindication about the waterband absorption, i.e. the thickness of themeasured layer. The reading of the measured thickness may be done bymeans of a calibration chart on which thicknesses are plotted as afunction of the number of digits displayed, but it will be understoodthat the apparatus can be arranged to provide, on an appropriatecalibrated scale, a direct reading of the measured thickness or of therelative deviation therefrom.

The output is linear with the measured thickness, because the wedge islinear in density and, as known, thickness and density are linearlyrelated to each other.

In case aging of the light source or the photo-cell, or any other changein the optical system causes changes in the output current of thephoto-cell, up to a given extent, the measurement result will not bealtered virtually since the variations in the reference and in themeasuring light beam equal each other so that comparison of thecorresponding electric signals leads to the same measuring result. Thisis illustrated by the two curves in dash and dot lines in FIG. 3 and itcan be seen in the graph that the horizontal line which is drawn throughthe new level of the reference signal at 1.71 pm intersects the curve ofthe measuring signal at 1.96 pm at a point which corresponds to the sameangular position of the filter disk.

The waveforms which have been shown in FIG. 3 are a theoretical approachonly and have been chosen instead of the actual signal representation inorder to facilitate the description of the apparatus.

Thus, it will be understood that the reference signal at 1.7 pm does nothave a square waveform as shown but instead thereof has a form whichrises and lowers more continuously as the filter moves into and out ofthe radiation path.

The measuring signal at 1.96 pm, generally, will not start near the zerolevel, but instead thereof will start at a value only slightly lowerthan the expected level of the reference signal and will end at a valueslightly higher. In this way the accuracy of the measurement is greatlyincreased although the thickness range which can be measured isrestricted thereby.

The following parameters illustrate the apparatus for one particularapplication.

Filter disk:

diameter 70 mm revolutions per second 50 Wedge:

linear continuous wedge range from 1.00 to 1.25 D

independent linearity 0.25 percent angular extension on the filter disk310 Impulses:

number of impulses over the measuring angle of3 l= 700.

Measurement:

range: x 20-30 pm H (dependent on the composition of the layer) accesstime is smaller than 20 msec.

resolving power I/700 accuracy 0.25

The calibration of the apparatus is done in practice by means of samplesthe thickness and the water content of which are exactly known. Thedesired measuring range is determined by the density range of the wedge29.

The desired measuring level, i.e. the nominal thickness, may bedetermined by a neutral grey filter which may be provided on the filterwhich determines the reference signal, or by adjusting the amount offeedback in the amplifier 45.

Another method of carrying out the calibration of the apparatus consistsin continuously measuring the thickness of a freshly coated layer thethickness of which is periodically increased and decreased with adetermined value by adjustment of the coating device.

When the frequency of the periodic changes is low, say one period isgreater than 100 seconds, and the variations of the adjustment of thecoating device are small, and the measuring results are obtainedaccording to known correlation techniques, this method leads tosatisfactory results.

In the construction of the apparatus according to the embodimentdescribed hereinbefore, one element may give rise to difficulties, viz.the interference filter for transmitting the wavelength of 1.96 pm. Saidfilter occupies a considerable area on the filter disk and the filtercharacteristics may not be uniform along the complete measuring angle.

An apparatus which is improved in this respect is described hereinafterwith reference to FIG. 7 wherein an arrangement is described forcarrying out the measurement in transmission and with reference to FIG.8 wherein the improved filter disk is shown and to FIG. which shows theoutput of the system.

The basic operation of the apparatus does not differ from that of theapparatus described hereinbefore since still a comparison is done of theattenuation of radiant energy at a first wavelength which is subject tosubstantial absorption by the medium with the attenuation of radiantenergy at a second wavelength which is substantially not subject toabsorption thereby, but the filter which detennines the mentioned firstwavelength, e.g. of 1.96 pm, now occupies the small angular part of thefilter disk, whereas the second wavelength is determined by a stationaryfilter with a relative broad band of 1.79 to 2.17 pm which includes alsothe first wavelength, and the corresponding greater angular part of thefilter disk carries no spectral filter but only the measuring wedge 29and a uniform neutral density filter (not shown) of a density of about1.5. The object of said neutral grey filter is to absorb the majorportion of radiation transmitted in the broad wavelength band of 1.79 to2.17 pm in order to make that the energy quantum transmitted in saidband is about equal to that which is transmitted in the narrow band of1.96 am. The electric signal which corresponds to the 1.96 pmwavelength, i.e. the measuring signal, is stored in the sample and holdamplifier circuit 43 whereas the measuring signal which corresponds tothe second wavelength and the value of which is a function of theposition of the wedge, is compared with the mentioned first electricsignal.

Referring to FIG. 7, the radiation from a source 10 is focused by a lens11 onto a mirror 12 and reflected thereby onto the lead-sulfidephoto-cell 22 through a focusing lens 15. Filters 23 and 24 determinethe transmitted radiation band which extends from 1.79 to 2.17 gm. Thefilter disk 16 is provided with an interference filter 50 whichtransmits the measuring beam at 1.96 pm and the boundaries of which aredetermined by absorbing sectors 26 and 27.

The remaining angular part of the disk has no specific spectralabsorption but is only provided with a continuous wedge 29 which isequal to that described in the apparatus according the first embodimentand with the mentioned neutral density filter.

In the operation of the apparatus, first the measuring beam at 1.96 pmis transmitted and the corresponding electric signal is stored in thecircuit 43. Thereafter the switch 41 is put in the lower position sothat the electric signal which is representative for the referenceradiation beam may be compared with the stored measuring signal.

It will be apparent that theoretically the described technique cannotlead to perfect results since the reference radiation beam, which shouldnot substantially be absorbed by the medium includes the 1.96 gmwavelength which is precisely subject to substantial absorption by themedium.

The sensitivity loss which is introduced in the described way in themeasurement is, however, negligible, since the variations in the amountof radiation which may occur at the wavelength of 1.96 pm are very smallin comparison with the amount of radiation which is transmitted in the1.79 to 2.17 pm band.

The measurement which was carried out by means of the describedapparatus covers an area of about 2 cm only of the measured layer.

In the practical application of the invention it is thus possible tomeasure the thickness of a layer being coated on a paper or film web ofconventional width, say 1.24 m, in the longitudinal direction bylocating the radiation source and the detector about centrally of theweb, or in the transverse direction by making the radiation source andthe detector to scan transversally of the web. In the mentioned way, alongitudinal, respectively a transverse thickness profile of the webcoating may be plotted.

Additionally, the apparatus according to the invention may be used inthe measurement of the thickness of two layers which are coated at thesame time or which are coated almost immediately one after another sothat there is no occasion of separately measuring the thickness of thefirst coated layer. The measurement of the distinct thickness of twosuch layers is possible when the spectral absorption, more particularlythe average infrared density, of both layers is different.

In the case of double coating of e.g. a light -sensitive emulsion layerand an antistress layer thereon, the measurement may occur as follows.

The average infrared density of the antistress layer is almost zero,since said layer contains substantially water.

The infrared density of the emulsion layer is relatively important sincethe infrared light is considerably difiused by the emulsion ingredients.

When thus the thickness of both layers is measured by 2 meters, e.g. aninfrared densitometer operating at a wavelength band from 0.9 to 1.1 m,and a waterband meter according to the present invention operating e.g.at 1.96 m, the waterband meter provides an indication of the thicknessof both said layers whereas the l.R.-meter indicates directly thethickness of the emulsion layer. The thickness of the antistress layermay be differentially calculated from both the measured values.

The invention is not limited to the described embodiments but otherelectronic circuits which may serve to indicate the angular position ofthe measuring wedge at the moment of equality of the signals from thereference and the measuring beams, other optical arrangements fordetermining the measuring radiation beam and the reference radiationbeam(s), etc., fall within the scope of the present invention.

1 claim:

1. A spectrophotometer wherein the measurement of the spectralabsorption of a medium occurs by comparison of the attenuation ofradiant energy at a first wavelength which is subject to substantialabsorption by the medium with the attenuation of radiant energy at asecond wavelength which is substantially not subject to absorptionthereby, comprising a. a radiant ener'gy'detector,

b. a radiant energy source for directing radiant energy in at least thementioned first and second wavelengths towards said medium so that saidenergy after impingement thereon is received on said detector,

c. filter means disposed in the radiation path between the source andthe detector and arranged to alternatively transmit radiation at saidfirst wavelength and radiation at least at said second wavelength,

d. a measuring wedge mounted for movement across the radiation beamwhile the radiation in one of said wavelengths is transmitted to thedetector for progressively altering the amount of transmitted radiantenergy as it moves through the radiation beam,

e. means for producing a first electric signal of a magnitudeproportional to the radiant energy received on the detector at thewavelength which is not attenuated by said measuring wedge and means forstoring said signal,

f. means for producing a second electric signal of a magnitudeproportional to the radiant energy received on the detector at thewavelength which is progressively altered by the said measuring wedge,

g. means for comparing said second electric signal with said first oneand for detecting when the two electric signals reach equal magnitude,and

h. means effective when said equality is detected for producing anindication of the position of the measuring wedge at said moment.

2. A spectrophotometer according to claim 1, wherein said filter meanscomprises a filter holder which is mounted for continuous rotationthrough the radiation beam and including a first filter transmittingradiation at said second wavelength which remains in the radiation beamfor a small angular displacement of the filter holder and a secondfilter transmitting radiation at said first wavelength which remains inthe radiation beam for a substantial angular displacement of the filterholder.

3. A spectrophotometer according to claim 1, wherein said filter meanscomprises a. a filter holder mounted for continuous rotation through theradiation beam, said holder carrying a first filter transmittingradiation at said first wavelength which remains in the radiation beamonly for a minor angular displacement of the filter holder, said filterholder transmitting for the remaining major part of its angulardisplacement all of the radiant energy of the source, and

b. a second filter fixed in the radiation beam which transmits radiationat said second and said first wavelength.

4. A spectrophotometer according to claim 3, wherein the limits of saidsecond wavelength extend almost symmetrically with said firstwavelength.

5. A spectrophotometer according to claim 2 wherein said measuring wedgeis mounted for introduction into the radiation-beam while the firstfilter, whichremains in the radiation beam over only a small angulardisplacement of the filter holder, is outside of the radiation beam.

6. A spectrophotometer according to claim 2, wherein said measuringwedge is mounted on said filter holder.

7. A spectrophotometer according to claim 1 wherein said means forproducing an indication of the position of the measuring wedge at themoment of equality of said two electric signals comprises:

a. a pulse generator,

b. a pulse counter, and

c. control means controlling said pulse counter to begin counting pulsesfrom said pulse generator at the moment the measuring wedge enters intothe radiation beam, to end said counting at the moment of equality ofthe electric signals and indicate the number of pulses counted as anindication of the wedge position, and to reset the counter to zero priorto reconnection to the pulse generator.

8. An infrared waterband meter for measuring the thickness of awater-containing coating applied to a substrate comprismg a. a sourcefor directing radiant energy at a measuring wavelength of L96 um and ata different reference wavelength which is substantially not absorbed bythe coating composition,

b. a detector for receiving the energy transmitted or reflected by thecoating after partial absorption thereby,

c. a circular filter holder carrying over a substantial angular partafilter transmitting only the 1.96 pm wavelength radiation and over asmall angular part a filter transmitting only the reference wavebandradiation, the filter transmitting the 1.96 pm wavelength radiationincluding a measuring wedge, and the filter holder being mounted forrotation to successively pass said filters through the radiation path,

d. means for producing a first electric output signal proportional tothe amount of radiant energy impinged at 1.96 pm measuring wavelength onthe detector, and means for storing said signal,

e. means for producing a second electric output signal proportion to theamount of radiant energy impinged at said reference wavelength on thedetector,

f. means for comparing said two output signals and for triggering avoltage comparator at the moment said output signals are equal,

g. a pulse generator,

h. a pulse counter,

i. and means for connecting said pulse counter to said pulse generatorat the moment the measuring wedge enters into the radiation beam, saidcounter being preset just prior to its connection in the circuit,disconnecting the counter from the pulse generator in response to thetriggering of said voltage comparator, and for re-setting said counterprior to reconnection to the pulse generator.

9. An infrared waterband meter for measuring the thickness of awater-containing coating applied to a substrate, comprismg a. aradiation source for directing radiant energy at a measuring wavelengthof 1.96 m and at a reference waveband from L79 to 2.17 am to thecoating,

b. a detector for receiving the energy which is transmitted or reflectedby the coating after partial absorption thereby,

c. a circular filter holder carrying over a small angular section afilter transmitting the 1.96 m wavelength, the remaining angular part ofthe filter holder showing no particular spectral absorption and ameasuring wedge of neutral density, said filter holder being mounted forrotation to successively pass said 1.96 pm filter and said measuringwedge through the radiation path,

d. a stationary filter in the radiation path which transmits radiationfrom 1.79 to 2.17 urn,

e. means for producing a first electric signal proportional to theamount of radiant energy impinged at said measuring wavelength on thedetector, and means for storing said signal,

f. means for producing a second electric signal proportional Y to theamount of radiant energy impinged at said reference wavelength on thedetector,

g. means for comparing the magnitude of said two signals and fortriggering a voltage comparator at the moment they reach equality,

h. a pulse generator,

i. a pulse counter,

j. and means for connecting said pulse counter to said pulse generatorat the moment the measuring wedge enters into the radiation beams, saidcounter being preset just prior to its connection in the circuit,disconnecting the counter from the pulse generator in response to thetriggering of said voltage comparator, and for re-setting said counterprior to reconnection to the pulse generator.

1. A spectrophotometer wherein the measurement of the spectralabsorption of a medium occurs by comparison of the attenuation ofradiant energy at a first wavelength which is subject to substantialabsorption by the medium with the attenuation of radiant energy at asecond wavelength which is substantially not subject to absorptionthereby, comprising a. a radiant energy detector, b. a radiant energysource for directing radiant energy in at least the mentioned first andsecond wavelengths towards said medium so that said energy afterimpingement thereon is received on said detector, c. filter meansdisposed in the radiation path between the source and the detector andarranged to alternatively transmit radiation at said first wavelengthand radiation at least at said second wavelength, d. a measuring wedgemounted for movement across the radiation beam while the radiation inone of said wavelengths is transmitted to the detector for progressivelyaltering the amount of transmitted radiant energy as it moves throughthe radiation beam, e. means for producing a first electric signal of amagnitude proportional to the radiant energy received on the detector atthe wavelength which is not attenuated by said measuring wedge and meansfor stoRing said signal, f. means for producing a second electric signalof a magnitude proportional to the radiant energy received on thedetector at the wavelength which is progressively altered by the saidmeasuring wedge, g. means for comparing said second electric signal withsaid first one and for detecting when the two electric signals reachequal magnitude, and h. means effective when said equality is detectedfor producing an indication of the position of the measuring wedge atsaid moment.
 2. A spectrophotometer according to claim 1, wherein saidfilter means comprises a filter holder which is mounted for continuousrotation through the radiation beam and including a first filtertransmitting radiation at said second wavelength which remains in theradiation beam for a small angular displacement of the filter holder anda second filter transmitting radiation at said first wavelength whichremains in the radiation beam for a substantial angular displacement ofthe filter holder.
 3. A spectrophotometer according to claim 1, whereinsaid filter means comprises a. a filter holder mounted for continuousrotation through the radiation beam, said holder carrying a first filtertransmitting radiation at said first wavelength which remains in theradiation beam only for a minor angular displacement of the filterholder, said filter holder transmitting for the remaining major part ofits angular displacement all of the radiant energy of the source, and b.a second filter fixed in the radiation beam which transmits radiation atsaid second and said first wavelength.
 4. A spectrophotometer accordingto claim 3, wherein the limits of said second wavelength extend almostsymmetrically with said first wavelength.
 5. A spectrophotometeraccording to claim 2 wherein said measuring wedge is mounted forintroduction into the radiation beam while the first filter, whichremains in the radiation beam over only a small angular displacement ofthe filter holder, is outside of the radiation beam.
 6. Aspectrophotometer according to claim 2, wherein said measuring wedge ismounted on said filter holder.
 7. A spectrophotometer according to claim1 wherein said means for producing an indication of the position of themeasuring wedge at the moment of equality of said two electric signalscomprises: a. a pulse generator, b. a pulse counter, and c. controlmeans controlling said pulse counter to begin counting pulses from saidpulse generator at the moment the measuring wedge enters into theradiation beam, to end said counting at the moment of equality of theelectric signals and indicate the number of pulses counted as anindication of the wedge position, and to reset the counter to zero priorto reconnection to the pulse generator.
 8. An infrared waterband meterfor measuring the thickness of a water-containing coating applied to asubstrate comprising a. a source for directing radiant energy at ameasuring wavelength of 1.96 Mu m and at a different referencewavelength which is substantially not absorbed by the coatingcomposition, b. a detector for receiving the energy transmitted orreflected by the coating after partial absorption thereby, c. a circularfilter holder carrying over a substantial angular part a filtertransmitting only the 1.96 Mu m wavelength radiation and over a smallangular part a filter transmitting only the reference wavebandradiation, the filter transmitting the 1.96 Mu m wavelength radiationincluding a measuring wedge, and the filter holder being mounted forrotation to successively pass said filters through the radiation path,d. means for producing a first electric output signal proportional tothe amount of radiant energy impinged at 1.96 Mu m measuring wavelengthon the detector, and means for storing said signal, e. means forproducing a second electric output signal proportion to the amount ofradiant energy impinged at said reference wavelength on the detector, f.means for comparing said two output signals and for triggering a voltagecomparator at the moment said output signals are equal, g. a pulsegenerator, h. a pulse counter, i. and means for connecting said pulsecounter to said pulse generator at the moment the measuring wedge entersinto the radiation beam, said counter being preset just prior to itsconnection in the circuit, disconnecting the counter from the pulsegenerator in response to the triggering of said voltage comparator, andfor re-setting said counter prior to reconnection to the pulsegenerator.
 9. An infrared waterband meter for measuring the thickness ofa water-containing coating applied to a substrate, comprising a. aradiation source for directing radiant energy at a measuring wavelengthof 1.96 Mu m and at a reference waveband from 1.79 to 2.17 Mu m to thecoating, b. a detector for receiving the energy which is transmitted orreflected by the coating after partial absorption thereby, c. a circularfilter holder carrying over a small angular section a filtertransmitting the 1.96 Mu m wavelength, the remaining angular part of thefilter holder showing no particular spectral absorption and a measuringwedge of neutral density, said filter holder being mounted for rotationto successively pass said 1.96 Mu m filter and said measuring wedgethrough the radiation path, d. a stationary filter in the radiation pathwhich transmits radiation from 1.79 to 2.17 Mu m, e. means for producinga first electric signal proportional to the amount of radiant energyimpinged at said measuring wavelength on the detector, and means forstoring said signal, f. means for producing a second electric signalproportional to the amount of radiant energy impinged at said referencewavelength on the detector, g. means for comparing the magnitude of saidtwo signals and for triggering a voltage comparator at the moment theyreach equality, h. a pulse generator, i. a pulse counter, j. and meansfor connecting said pulse counter to said pulse generator at the momentthe measuring wedge enters into the radiation beams, said counter beingpreset just prior to its connection in the circuit, disconnecting thecounter from the pulse generator in response to the triggering of saidvoltage comparator, and for re-setting said counter prior toreconnection to the pulse generator.